If a female is ready for spawning, yet e.g. this doesn't happen because the pair doesn't like each other:

What are the consequences for the female when the eggs "turn old"?- Will the body break them down?
- Or could this potentially result in problems for the female? -> if so, how to remediate the problem?

As far as I know the female will just absorb the eggs back into her body so its ok if they dont spawn. I've looked at your other post and I honestly have no idea. I'm sorry to hear about it though. I really do hope she gets better

Don't worry about your female. They will either drop and eat her eggs or absorb them without any negative consequences.

The illness is (I think - not too good on diseases) bacteria related due to bad water. If not severe (hasn't spread), AQ salt should help. But anti bacterial medication is better. If most of the belly is redish specially with bloating, it usually can't be saved.